env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/kosagi/novena/novena.c b/board/kosagi/novena/novena.c
index dcf3d7f..e0f0d7c 100644
--- a/board/kosagi/novena/novena.c
+++ b/board/kosagi/novena/novena.c
@@ -9,6 +9,7 @@
 #include <dm.h>
 #include <dm/device-internal.h>
 #include <ahci.h>
+#include <env.h>
 #include <linux/errno.h>
 #include <asm/gpio.h>
 #include <asm/io.h>